The demand for films of higher sensitivity with reduced granularity dictates the need to make more sensitive silver halide emulsions without increasing the size of the silver halide crystals that make up the emulsion. This requires the crystals to be designed and sensitized so that they require fewer photons to render them developable by an appropriate developing agent. This is commonly referred to as increased photoefficiency.
Presently, emulsions of high photoefficiency can be produced from a wide variety of grain morphologies and halide structures by treating them with a temperature cycle in the presence of compounds that liberate sulfur and gold on the surface of the silver halide crystals. This process is commonly referred to as sulfur and gold chemical sensitization and is well known in the art. The sulfur and gold sensitization is often done in the presence of spectral sensitizing dyes which induce sensitivity at longer wavelengths than the silver halide can intrinsically absorb. These sensitizing dyes, along with other heterocyclic aromatic compounds, also influence the chemical sensitization process by controlling fog and allowing higher sensitivity to be achieved without high fog.
Most light recording silver halide used for photography are deficient in blue light sensitivity relative to green and red light sensitivity, and thus obtaining the high blue light sensitivity that is needed for camera speed materials with ISO film speeds of 400 or greater will demand large silver halide crystals sensitized to the limit of available technology. Currently these extremely high blue sensitivities are achieved by low aspect ratio twinned emulsions with high bulk iodide levels and often contain internal phases that are near the saturation limit of iodide in the bromide face centered cubic lattice. Recently high aspect ratio emulsions with low bulk iodide levels have been used in blue sensitive layers of high speed negative film. These emulsions are sensitized with traditional sulfur plus gold sensitizations which include blue sensitizing dye at levels proportional to the specific surface area of the emulsion.
Acetylenic derivatives of 2-aminobenzoxazole are highly effective latent image stabilizers according to Lok et al. in U.S. Pat. Nos. 4,378,426 and 4,451,557. Fresh speed increase was also indicated, but the primary advantage was for latent image keeping (LIK), where the acetylenic compounds are preferably added during the coating process, although addition during the finish is suggested. These results and conclusions were based on a non-spectrally sensitized AgBr octahedra.